Feynman – How to Move the Position of Labelling in Feynmp

feynmanfeynmp-auto

What if I want label on other side of vertex or fermion?

    \begin{center}
    \begin{fmffile}{elecdig}
    \begin{fmfgraph*}(150,100)
    \fmfpen{thin}
    \fmfleft{i1,i2,ip,i3,i4} 
    \fmfright{o1,o2,op,o3,o4}
    \fmfpen{thick}
    \fmf{fermion,tension=1,label=$v$}{i3,o3}
    \fmf{fermion,tension=1,label=Spin Direction}{i4,o4}
    %\fmf{fermion,tension=1,label=$spin=+\frac{1}{2}$}{v2,o2}
    \fmf{dashes,label=It can't be ${\hat{e}_R}$ but it is $e_R$}{ip,op}
    \fmf{phantom,tension=1}{o2,o1}
    \fmf{phantom,tension=1}{i2,i1}
    \fmf{fermion,tension=1,label=$v_{new}$}{o2,i2} 
    \fmf{fermion,tension=1,label=Spin Direction}{i1,o1}
    %\fmf{fermion,tension=1,label=$spin=+\frac{1}{2}$}{v1,o1}
    \fmflabel{Frame $1$.}{i3}
    \fmflabel{Frame $2$.}{i2}
    \end{fmfgraph*} 
    \end{fmffile}
    \end{center}

I want to move v_new to another side
There is also a problem when I am not using \fmfpen{thick or thin} and the second line from above automatically thin compare to others.
i.e.,

    \begin{center}
    \begin{fmffile}{elecdig}
    \begin{fmfgraph*}(150,100)
    %\fmfpen{thin}
    \fmfleft{i1,i2,ip,i3,i4} 
    \fmfright{o1,o2,op,o3,o4}
    %\fmfpen{thick}
    \fmf{fermion,tension=1,label=$v$}{i3,o3}
    \fmf{fermion,tension=1,label=Spin Direction}{i4,o4}
    %\fmf{fermion,tension=1,label=$spin=+\frac{1}{2}$}{v2,o2}
    \fmf{dashes,label=It can't be ${\hat{e}_R}$ but it is $e_R$}{ip,op}
    \fmf{phantom,tension=1}{o2,o1}
    \fmf{phantom,tension=1}{i2,i1}
    \fmf{fermion,tension=1,label=$v_{new}$}{o2,i2} 
    \fmf{fermion,tension=1,label=Spin Direction}{i1,o1}
    %\fmf{fermion,tension=1,label=$spin=+\frac{1}{2}$}{v1,o1}
    \fmflabel{Frame $1$.}{i3}
    \fmflabel{Frame $2$.}{i2}
    \end{fmfgraph*} 
    \end{fmffile}
    \end{center}

this gives,

enter image description here

I am restating the problem with reference to this image:

  1. 2nd line from above is thin compare to others

  2. I don't know how to shift label $v_{new}$ to other side.

Best Answer

Here's an alternative answer that doesn't use feynmf, but uses a new package called TikZ-Feynman (CTAN). TikZ-Feynman has inbuilt support for momentum arrows which, in this case, can be used to indicate the velocity and spin direction of fermions:

\documentclass{article}

\usepackage[compat=1.1.0]{tikz-feynman}

\begin{document}\noindent
\feynmandiagram [horizontal=a to b] 
                {a -- [fermion, momentum=\(v\), momentum'=spin] b}; \newline
\feynmandiagram [horizontal=a to b] 
                {a -- [fermion, reversed momentum=\(v\), momentum'=spin] b}; \newline
\end{document}

output

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